This invention relates to a steering system for a vehicle, and more particularly but not exclusively to a steering system for a vehicle which is used both on and off-road, such as for example, a tractor.
Until recently it used to be a legal requirement in the UK, for any vehicle which is capable of high speed, which travels on public roads, to have mechanical steering in which there is a steering control which is mechanically connected via a mechanical steering train to one or more steerable wheels of the vehicle, for safety reasons, so steering capability is not lost at speed, in the event of any power failure.
Tractors are known which are capable of travelling at high road speeds. These provide considerable advantage to e.g. farmers, who are able to move between agricultural work sites quickly, without delaying other road traffic. Hithertofore, these have had mechanical steering systems.
Modern tractors are expected to be able to perform agricultural tasks, such as crop spraying, using automated systems which are required to be more responsive than a mechanical system tends to be. For example only, it is known to control a tractor with hydraulically powered steering, automatically using an electronic controller which determines the tractor's position e.g. using a GPS. The electronic controller may be programmed with a map of a field for example, where the tractor is to perform spraying, so that in automatically guiding the tractor to perform spraying, the tractor can be maintained within the field boundary (or boundary within a field) and any obstacles such as trees, pylons and the like can be navigated around.
Hydraulically powered steering systems provide the required responsiveness for automated steering, but for the reasons set out above, have not been used on tractors which are capable of high on-road speeds.